The present invention is directed in general to double band presses for the continuous production of endless material webs and, in particular, to a new and useful double band press which uses the passage of current through the band to heat the material web.
Double band presses of this type (DE-OS 24 21 296) serve particularly for the fabrication of decorative plastic laminates, chipboard, fiberboard, plywood panels, copper-coated electrolaminates which are the initial material for printed circuit boards, thermoplastic webs and other laminated materials.
To obtain a good quality final product, it is necessary to cure the binder (synthetic resin) contained in the material to be molded under pressure during the passage of the material through the double band press and, simultaneously, to transfer the quantity of heat required for curing to the material to be molded. Since the heated press bands have only a certain heat capacity, the maximum amount of heat which can be transmitted to the molding material is limited and insufficient for many applications. The supply of additional heat through heating or pressure plates and through fluid pressure means is limited by the bad heat conductivity of fluid media.
It is indeed known to preheat the molding material already before entry into the double band press, for instance, by infrared radiation. However, this can result in an undesirable precondensation of the resin contained in the molding material so that an end product of a lower quality is formed. Furthermore, the energy costs are uneconomically high in this type of heat transmittal and no predetermined temperature gradient can be produced by this measure in the molding material during the pressing or extrusion process, as this is advantageous in some applications.
It is known from DE-OS 33 25 578 to supply heat in the reaction zone of a double band press by arranging elements of a material with good heat conductivity in the pressure equalizing plate of the double band press. These elements are connected to the heating or pressure plate so as to have a good heat transmission contact and contact the inner side of the press band in a dragging or sliding manner. It is however disadvantageous here that these elements must be pressed with a specific force against the inner surface of the band in order to achieve a good heat transmission contact, whereby the tensile forces applied to the press band increase. In order to be able to transmit the heat quantities required in actual practice, provision of a large quantity of such heat conducting elements is necessary so that, thereby, the maximum allowable band stresses in the press band can be exceeded. In addition, there exists the danger of destruction of the surface of the press band by the frictional forces arising between the band's inner surfaces and the heat conducting elements pressed against them, which frictional forces arise in the course of the dragging or sliding motion of the press bands along the heat conducting elements.